1. Field of the Invention
The present invention relates to a substrate for forming a solid-state image pickup element, a solid-state image pickup element using the substrate, and a method of producing the substrate, and more particularly to an improvement in the performance of a solid-state image pickup element.
2. Description of the Related Art
A solid-state image pickup element using a CCD which is employed in an area sensor or the like has: a photoelectric converting section configured by photodiodes or the like; a charge transferring section having charge transfer electrodes which transfer signal charges supplied from the photoelectric converting section; and a wiring section connected to these components. The charge transfer electrodes are adjacently placed on a charge transfer path formed on a semiconductor substrate, and sequentially driven.
Recently, in accordance with an increased number of pixels of a CCD, requests for a higher resolution and a higher sensitivity are growing in a solid-state image pickup element The number of imaging pixels is increased to gigapixels or more.
Under such circumstances, in order to ensure a high sensitivity, since it is difficult to reduce a light receiving area, an area occupied by charge transfer electrodes is forced to be reduced.
Under such circumstances, also requests for improving the performance of a semiconductor substrate for forming a solid-state image pickup element are increasing.
As a semiconductor substrate for forming a solid-state image pickup element, a CZ substrate grown by the Czochralski (CZ) method, an MCZ substrate grown by the magnetic Czochralski (MCZ) method, an epitaxial semiconductor substrate in which an epitaxial layer is formed on the surface of such a CZ or MCZ substrate, and the like are conventionally used.
On the other hand, today, a step of forming a semiconductor device is performed in a super clean room having a very high cleanliness. However, contamination of a semiconductor substrate due to impurities from gasses, water, a semiconductor producing apparatus, and the like cannot be completely avoided. The amounts of impurities which are introduced into a semiconductor substrate in a step of forming an epitaxial layer on the surface of the semiconductor substrate are said to be larger than those of impurities which are introduced in a step of forming a semiconductor device.
When an impurity or a crystal defect exists in an element active region of a semiconductor substrate, the quality and characteristics of a semiconductor device are significantly deteriorated. When an impurity or a crystal defect exists in a semiconductor substrate, the semiconductor substrate is susceptible to an irradiation damage due to radiation such as α rays. This damage causes the quality and characteristics of a semiconductor device to be further deteriorated.
In order to remove away such an impurity or a crystal defect from an element active region, intrinsic gettering (IG) based on oxygen precipitation from the interior of a substrate, and extrinsic gettering (EG) not involving oxygen precipitation are conventionally performed (for example, JP-A-2003-92301).
For example, the intrinsic gettering process is performed by executing a two-step heat treatment on a silicon wafer in which an epitaxial growth layer is formed. The two-step heat treatment is configured by an outward diffusion heat treatment in which the wafer is heated in a high-temperature oxygen atmosphere, and a heat treatment for forming a low-temperature oxygen precipitation nucleus. In this way, the intrinsic gettering process is a treatment in which an oxygen precipitation defect is used as the medium. Therefore, it is the that, when the dopant concentration of a substrate exceeds 1018 cm−3, the gettering effect is suppressed in an N+ substrate (for example, “Silicon no kagaku” UCS Handotai Kiban Gijutsu Kenkyukai, pp. 602-607).
Under the situations, as miniaturization of a solid-state image pickup element is further advancing, occurrence of a white scratch defect becomes more problematic. A white scratch defect corresponds to a dark current caused by an impurity or the like, and hence gettering is indispensable. In the case where a high-temperature step is necessary, however, a special apparatus may be required, or a problem of warpage or distortion of a substrate may become more serious on the contrary.